In-Solution Fluorescence

The solution fluorescence properties of 21-28 were also investigated. As anticipated, the increased dynamic and rotation of molecules in solution leads to considerably different fluorescence properties than in the solid state. In general, the fluorescence of 21-28 in solution is weak. In 2001 Yip reported on fluorescence quenching of a phosphane substituted anthracene derivative, ascribing the observed suppression of emission to rapid electron transfer from the electron rich diphenylphosphanyl substituent to excited fluorophore.^[65][x]

Figure 3-49: Excitation (green) and emission (red) spectra of MeAnPS(NEt2)2 (23) (left) and [MeAnP(NEt2)2(S)AuCl] (24) (right), 5∙10^-5 M in dichloromethane.

Although the phosphorus atom has neither a lone-pair nor is substituted by phenyl groups in 21-28, electron transfer is nevertheless the cause of the weak fluorescence discussed vide infra.

Alike in solid state, the emission in solution of all selenium oxidized compounds is by about the factor of 10 weaker than of the sulfur oxidized derivatives. Thus only molecules containing the same chalcogens will be compared to one another. For the sulfur oxidized compounds 21-24 a distinct red shift of emission is observed for both gold complexes compared to the free ligands. Figure 3-49 illustrates this bathochromic shift for MeAnPS(NEt2)2 (23) and the corresponding gold complex [MeAnP(NEt2)2(S)AuCl] (24). While for the free ligand the excitation and emission maxima are in close proximity (~45 nm) they separate by nearly 100 nm upon complexation of gold(I). While the excitation maximum is only slightly shifted, the emission maximum is red shifted by over 50 nm. Also the vibrational structure of the free ligand’s emission band is not present in the emission spectrum of the gold complex which shows a single broad emission band. A similar shift is also observed for MeAnPS(NMe2)2 (21) and its gold complex [MeAnP(NMe2)2(S)AuCl] (22), although not as strong as for 23 and 24.

Figure 3-50: Emission spectra of MeAnPS(NMe2)2 (21) (blue), [MeAnP(NMe2)2(S)AuCl] (22) (cyan), 23 (red) and [MeAnP(NEt2)2(S)AuCl] (24) (green), 5∙10^-5 M in dichloromethane.

While the emission intensities of 22, 23, and 24 are nearly identical and observed differences can well be ascribed to small deviations of sample concentrations, MeAnPS(NMe2)2 (21) shows notably weaker emission. This can be attributed to electron transfer processes. Alkyl amines are known to be efficient quenchers of anthracene fluorescence.^[12-13]

This phenomenon has been exploited in many sensor molecules which carry receptor moieties with amino groups as quenchers, usually linked to the anthracene fluorophore by methylene spacers.^[23c] They have delivered the most effective quenching rates compared to longer spacers in systems of this type.^{[13, 22]} This is due to the optimum arrangement achieving maximum orbital overlap facilitating electron transfer to the excited fluorophore.^[13] In most cases the methylene spacer carries only one amino group and the required orbital overlap is promoted by rapid rotation around the single bonds of the spacer, resulting in a faster electron transfer than the lifetime of the excited state. In the case of 21-28 two amino groups are present in a geometric arrangement very similar to those of the methylene spacer (Scheme 3-7).

Table 3-14: Solution fluorescence of 21-24.

21 22 23 24

λEx (max) [nm] 397 397 400 411 λEm (max) [nm] 469 504 434 508

Irel 0.29 0.81 1 0.90

Scheme 3-7: Geometrical analogies of methylene bridged amine quenchers and amino substituents in 21-28.

Obviously the presence of two quenchers leads to an even higher probability of orbital overlap and electron transfer to the excited fluorophore. Hence, quenching is quite effective and the emission of 21-28 is weak. In contrast to sensor molecules made for metal ion detection, the energy of the amino-lone pairs is not lowered in the gold complex, because only the soft chalcogen donors participate in complexation and not the harder nitrogen donors. Therefore the quenching mechanism is not influenced.

The only remaining factor to modify the emission intensity is the rate of rotation about the An-P and P-N bonds. A higher rate of rotation would lead to a higher probability of orbital overlap and consequently to effective quenching. As the steric bulk of the phosphoryl substituent should affect the rate of rotation of the same, this should also become apparent in the effectiveness of quenching.

MeAnPS(NMe2)2 (21), the compound with the smallest substituent, indeed shows the weakest emission. In MeAnPS(NEt2)2 (23) with a distinctly bulkier substituent and lower rotation rates the quenching is less effective. Both gold complexes [MeAnP(NMe2)2(S)AuCl] (22) and [MeAnP(NEt2)2(S)AuCl] (24) show similar emissions to 23. In the case of 24 this is not surprising, as it also carries bulky diethylamino groups.

The less effective quenching of 22 can only be attributed to the complexation of gold(I) which should also notably hinder rotation of the phosphoryl substituent. Proof of a sufficient interaction between ligand and gold ion is given by the strong red shift of emission for both gold complexes. The observed phenomena follow the hypothesis of reduced quenching by steric inhibition of rotation of the phosphoryl substituent.

Figure 3-51: Excitation (green) and emission (red) spectra of MeAnPSe(NEt2)2 (27) (left), and emission spectra of MeAnPSe(NMe2)2 (25) (blue), [MeAnP(NMe2)2(Se)AuCl] (26) (cyan), MeAnPSe(NEt2)2 (27)

(red), and [MeAnP(NEt₂)₂(Se)AuCl] (28) (green) (right), all 5∙10^-5 M in dichloromethane.

The selenium oxidized compounds 25-28 show several similarities to 21-24 regarding their in-solution fluorescence properties. The most important finding is that the free ligands MeAnPSe(NMe2)2 (25) and MeAnPSe(NEt2)2 (27) follow the trend established for their sulfur oxidized counterparts, showing very similar emission proportions as 21 and 23. Again the compound which carries the less bulky phosphoryl substituent exhibits more effective fluorescence quenching. In contrast, the gold complexes [MeAnP(NMe2)2(Se)AuCl] (26) and [MeAnP(NEt2)2(Se)AuCl] (28) do not show the characteristic red shift of emission observed for [MeAnP(NMe2)2(S)AuCl] (22) and 24.

This can be explained by the weak Se-Au-interaction which most likely leads to a complete dissociation of the Au-Se-contact in solution. Additionally the general fluorescence quenching effect of the selenium atom in the molecular structure which has already been documented in terms of solid state fluorescence appears to be intensified by the presence of a second heavy atom. This leads to very weak emission which lets the results obtained for 26 and 28 appear less significant.

In retrospect, it was possible to trace the coordination of gold(I) to MeAnPS(NMe2)2

(21) and MeAnPS(NEt2)2 (23) in solution by the means of fluorescence emission. The red shift of the emission maximum by well over 50 nm enforced by the coordination of

Table 3-15: Solution fluorescence of 25-28.

25 26 27 82

λEx (max) [nm] 382 384 381 384 λEm (max) [nm] 439 467 432 464 Irel 0.50 0.24 1 0.14

gold(I) is a significant and well measurable effect. Though overall weakly fluorescent, this behavior upon metal coordination makes this compound class interesting for development of potential sensor systems based on colorimetric detection, rather than typical on/off switching. Furthermore a correlation of emission properties with the steric demand of the phosphoryl substituent was found which lead to a promising hypothesis on rotational rates and their influence on fluorescence quenching.

3.4 Synthesis of new Phosphoryl Anthracenes and